Electrical-discharge-inhibiting conformable layer for use in inner-cooled coils

ABSTRACT

A conformable layer ( 14 ) for inhibiting electrical discharge between vent tubes ( 16 ) and strands ( 12 ) in an inner-cooled coil ( 5 ). The conformable layer comprises a resistive inner core ( 24 ) and a conductive strip ( 20 ) wrapped in a conductive outer wrap ( 26 ). The conductive strip ( 20 ) is electrically connected to the strands ( 12 ) at one end of the coil ( 5 ) and left to electrically float at the other end. In this configuration, the conformable layer ( 14 ) reduces voltage buildup between the vent tubes ( 16 ) and the strands ( 12 ) to help prevent electrical damage to the coil ( 5 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.11/087,002, filed Mar. 22, 2005.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

SEQUENCE OR COMPUTER PROGRAM LISTING

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to high-voltage stator coils and moreparticularly to methods and apparatuses for inhibiting electricaldischarge between vent tubes and strands in inner-cooled stator coils.Although the following discussion focuses on stator coils forturbogenerators, the present invention is applicable to otherdynamoelectric machines, including electric motors.

Conventional turbogenerators have a rotor and a stator. The rotor iswound with field windings, which are disposed in slots in the body ofthe rotor. The stator is wound with stator coils, which are disposed inslots in the body of the stator. When the rotor is rotated by anexternal source of mechanical energy, such as a steam turbine or a gasturbine, and an excitation current is provided to the field windings,electrical energy is induced in the stator coils.

Stator coils are generally constructed from a plurality of individualconductors referred to as strands. The strands are stacked together toform a larger conductor (or coil) capable of carrying high voltages andcurrents. In many stator coils, the strands are twisted into a weavedpattern rather than simply being stacked one on top of another. Thisweaving technique is known as Roebelling. elling helps prevent the innerstrands of a stator coil, which are closest to the rotor, from carryingmore current (and generating more heat) than the outer strands, whichare further from the rotor. elling helps ensure that each strand carriesa similar amount of current and generates a similar amount of heat.

Some stator coils include integral vent tubes to help cool the strands.These types of stator coils are referred to as inner-cooled coils. Ininner-cooled coils, a plurality of vent tubes are generally stacked ontop of one another and sandwiched between two or more stacks of strands.A cooling gas like hydrogen or air is then pumped through the vent tubesto help transfer heat away from the strands.

There are a number of challenges associated with manufacturinginner-cooled stator coils. For example, after a stack of strands hasbeen elled, the top and bottom surface of the stack is no longer smooth.The surfaces have significant irregularities or indentations caused bythe elling of the strands. These irregularities make it difficult toapply the outer layer of insulation, referred to as ground-wallinsulation.

Another challenge involves the fact that an extremely large voltagedifferential can appear between the strands and the vent tubes in astator coil while a generator is operating. If this voltage differentialexceeds the dielectric strength of the insulation between the strandsand the vent tubes, an electrical short will occur between the copperstrands and the vent tubes, which can lead to circulating currents inthe vent tubes and catastrophic damage to the stator coil.

In an effort to inhibit electrical shorts between stands and vent tubes,U.S. Pat. No. 6,624,547 to Emery, which is incorporated by referenceherein in its entirety, discloses reducing the potential differencebetween copper strands and vent tubes by introducing a compact voltagegrading means between the copper strands and the vent tubes. Thisgrading means is formed from conductive strips that are positionedbetween a stack of strands and a stack of vent tubes. The conductivestrips are fixed in place with multiple layers of insulating tape andprovide electrical grading between the strands and vent tubes. Despitethe significant advancement provided by this approach, there remains acontinued need for improvements in stator coil configurations thanprovide increased protection against electrical shorts, while alsoreducing the complexity and costs associated with manufacturing statorcoils.

SUMMARY OF THE INVENTION

With the foregoing in mind, methods and systems consistent with thepresent invention provide a conformable layer that inhibits electricalshorts between vent tubes and strands in an inner-cooled coil. Theconformable layer conforms to the irregular surface of a stack of venttubes and strands in order to facilitate the application of groundwallinsulation over the stator coil. The conformable layer includes aconductive strip for electrically connecting the conformable layer to acorresponding stack of strands in order to reduce the potentialdifference between the strands and the vent tubes and thereby inhibitingelectrical shorts there between.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail by way of example withreference to the following drawings:

FIG. 1 illustrates a stator coil consistent with an exemplary embodimentof the present invention.

FIG. 2 illustrates an electrical-discharge-inhibiting conformable layerconsistent with an exemplary embodiment of the present invention.

FIG. 3 illustrates a schematic diagram of the stator coil of FIG. 1.

FIG. 4 illustrates a second schematic diagram of the stator coil of FIG.1.

FIG. 5 illustrates tube-to-copper voltage in a stator coil with andwithout electrical-discharge-inhibiting conformable layers.

DETAILED DESCRIPTION

Methods and systems consistent with the present invention provide anelectrical-discharge-inhibiting conformable (EDIC) layer for use ininner-cooled coils. Although the invention is described below inconnection with a generator stator coil, methods and systems consistentwith the invention are suitable for use with other dynamoelectricmachines, including motors. The invention may also be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. The scopeof the invention should be determined based upon the appended claims andtheir legal equivalents.

FIG. 1 illustrates a stator coil 5 consistent with an exemplaryembodiment of the present invention. A complete generator stator coil isgenerally manufactured from a number of individual stator coil sections.These sections include a straight portion and an involute portion (notshown). The straight portion is approximately the same length as thestator slot for which it is intended. The involute portions are locatedat each end of the straight portion and facilitate the interconnectionof multiple stator coil sections into a complete generator stator coil.

The stator coil 5 illustrated in FIG. 1 is an inner-cooled stator coil.It includes a stack of vent tubes 16 and a stack of strands 12. The venttubes 16 are preferable substantially rectangular and tubular andconstructed of a conductive material such as copper. Multiple vent tubesmay be stacked on top of one another for increased cooling capacity.Each of the vent tubes 16 preferably includes a layer of insulation overits outer surface (not shown). The stator coil 5 illustrated in FIG. 1also includes four stacks of strands 12. The strands 12 are preferablysubstantially rectangular and solid and constructed of a conductivematerial such as copper. The strands 12 may be weaved into a el patternfor increased efficiency. Each of the stands 12 preferably includes alayer of insulation over its outer surface (not shown).

An EDIC layer 14 is installed on the upper and/or lower surface of thestack of strands 12 and vent tubes 16. The EDIC layer 14 conforms toirregularities in these surfaces to facilitate the application ofgroundwall insulation 18. An outer electrode 10 is applied over thegroundwall insulation 18. The length of the EDIC layer 14 is preferablysubstantially equal to that portion of the stator coil 5 that includesan outer electrode 10. The width of the EDIC layer 14 is preferablysubstantially equal to the width of the corresponding stack of strands12 and vent tubes 16. The thickness of the EDIC layer 14 is preferablysubstantially equal to the depth of the irregularities in the surface ofthe elled strands.

FIG. 2 illustrates an EDIC layer 14 consistent with an exemplaryembodiment of the present invention. When referring to the surfaces ofthe EDIC layer, the surface closest to the strands 12 and vent tubes 16will be referred to as the inner surface. The opposite surface will bereferred to as the outer surface. The EDIC layer 14 includes an innercore 24, a conductive strip 20, and an outer conductive wrap 26. Theinner core 24 comprises a substantially resistive material, such asaramid reinforcement fiber impregnated with 25-60% B-staged epoxy resinbinder. One example of a suitable reinforcement fiber is Nomex™ spunfelt, available from E.I. Du Pont de Nemours & Co., Inc. One example ofa suitable binder is novolak thermoplastic B-staged phenolic resin. Theconductive strip 20 of the EDIC layer 14 is positioned adjacent to theinner core 24. The conductive strip 20 may be constructed of anyconductive material, such as metal, but is preferably constructed ofcopper. The conductive strip also preferably has a surface resistance ofless than 1 ohm/square. The EDIC layer also includes a conductive outerwrap 26. The conductive outer wrap 26 is wrapped around the inner core24 and conductive strip 20. In the preferred embodiment, the wrap 26 isformed from a single sheet of conductive fleece, such as carbon loadedglass or polyester fleece. The conductive outer wrap 26 preferably has awidth slightly larger than the perimeter of the inner core 24 so that itmay be wrapped around the inner core 24 and conductive strip 20 and forma small lap joint 22.

It will be understood by those skilled in the art that EDIC layersconsistent with the present invention may be produced using variousmanufacturing methods. One example of a method suitable for forming theEDIC layer 14 comprises the steps of (a) taking a layer of aromaticpolyamide (or aramid) reinforcement felt, (b) positioning a conductivestrip adjacent to the felt, (c) wrapping the felt and strip with aconductive outer wrap, (d) impregnating the assembly with an epoxyresin, (e) draining the excess resin, and (f) pressing and baking theassembly in a heated press to cure it to a B-stage. While the assemblypreferably starts out with a thickness of approximately 130 mils, afterit is pressed and baked it will be compressed by approximately 20-80%.In addition while the starting surface resistivity of the assembly maybe as low as 50-100 ohms/square, after pressing the assembly shouldpreferably have a surface resistivity of 2000-5000 ohms/square.

The resulting EDIC layer may be installed on either the top or bottomsurface of a stack of strands and vent tubes but is preferably installedon both the top and bottom surface. During installation, one end of theconductive strip is electrically connected to the strands via welding,brazing, or soldering and the other end is left to electrically float.Approximately ¼ square inch of the strand's surface may be stripped ofinsulation to facilitate the electrical connection. In a preferredembodiment, the conductive strip is positioned on the outer surface ofthe EDIC layer to make it easier to access during installation.

FIGS. 3 and 4 are electrical schematics illustrating the electricalproperties of a stator coil consistent with the present invention.Capacitance C1 represents the capacitance between the strands 12 and theouter ground electrode 10. Capacitance C2 represents the capacitancebetween the strands 12 and the vent tubes 16. Capacitance C3 representsthe capacitance between the outer ground electrode 10 and the EDIC layer14. Due to the addition of the EDIC layer 14, an additional capacitanceC4 is present between the EDIC layer 14 and the vent tubes 16. Voltage30 represents the applied voltage across the stands 12 and outer groundelectrode 10. The resistance of the EDIC layer 14 is represented byresistance R, which acts to reduce voltage buildup across the strands 12and vent tubes 16. The additional capacitive reactance of C4 also helpsto reduce voltage buildup between the strands 12 and vent tubes 16. Thevalue of resistance R is preferably selected to optimally reduce thevoltage across C2 and C4.

FIG. 5 is a graph illustrating exemplary results from electrical testsperformed on stator coils with and without EDIC layers. The graphrepresents the voltage (labeled Tube-to-Copper Voltage) between theouter vent tube (i.e., the top or bottom vent tube in a stack of venttubes) and the strands in a test coil as a function of the voltage onthe strands (labeled Applied Coil Voltage). Two tests are illustrated inFIG. 5. The first test (identified by triangles) involved a coil with noEDIC layers installed. In this configuration, an Applied Coil Voltage ofapproximately 5 kilovolts (rms) resulted in a Tube-to-Copper Voltage ofapproximately 250 volts (rms). The second test (identified by diamonds)involved a coil with an EDIC layer installed on the top and bottom ofthe stack assembly of the coil. In this configuration, an Applied CoilVoltage of approximately 5 kilovolts resulted in a Tube-to-CopperVoltage of approximately 25 volts (rms). These results verify thatTube-to-Copper Voltage can be significantly reduced by including EDIClayers in a coil.

1. A conformable layer for inhibiting electrical discharge between avent tube and a strand in a stacked coil assembly, comprising: an innercore including a conductive strip for electrically connecting saidconformable layer to said strand, and an outer conductive layer formedaround said inner core and said conductive strip.
 2. The conformablelayer of claim 1, wherein said inner core comprises a composite formedfrom an aramid fiber reinforcement impregnated with an epoxy resinbinder.
 3. The conformable layer of claim 2, wherein said composite ispressed and baked to form a cured conformable layer having a resistanceof 2000-5000 ohms/square.
 4. The conformable layer of claim 2, whereinsaid cured conformable layer has a thickness of 26-104 mils.
 5. Theconformable layer of claim 2, wherein said outer conductive layercomprises a sheet of conductive fleece that is wrapped around said innercore and said conductive strip.
 6. The conformable layer of claim 5,wherein said aramid fiber reinforcement comprises NOMEX spun felt. 7.The conformable layer of claim 6, wherein said epoxy resin bindercomprises novolak thermoplastic B-staged phenolic resin.
 8. Theconformable layer of claim 7, wherein said conformable layer is sized tohave a length and width substantially equal to the length and width ofsaid stacked coil assembly and a thickness substantially equal to thedepth of irregularities in the surface of said stacked coil assembly.